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This study aimed to evaluate the production of hydrogen in an upflow anaerobic fixed-bed reactor (UAFBR) fed with undiluted and non-nutrient supplemented cassava starch wastewater (CSW), simulating a scenario verified in the industry, in which variations in the organic loading rate (OLR) can occur as a function of the influent composition. To assess the effective behavior of the fermentative process, considering the reactor resilience facing the variations in the OLR and compositional characteristics of the substrate, the experimental conditions were delineated according to the carbohydrates (Carb) content of the CSW samples collected weekly, maintaining the integral composition of the residue. Seven assays were carried out with hydraulic retention time (HRT) of 4 and 3 hours and OLR of 15-35 gCarbL À1 d À1 . Maximum hydrogenogenic activity was obtained with the application of an OLR of 26 gCarb L À1 d À1 and HRT of 4 hours, with productivity and yield values of 0.7 LH 2 L À1 d À1 and 1.2 molH 2 kg À1 TCarb, respectively. Hydrogen evolution was marked by operational instability, with large variations in the volumetric production (>50%) in a short period of continuous production (<45 days). Despite the strategies adopted to support adequate substrate concentration in the reactor, that is, low HRT and high OLR, the unbalanced concentrations of nitrogen, phosphorus, and divalent cations (relative to the organic content) favored enhanced cell growth, decreasing carbohydrate availability, and compromising hydrogen evolution. To overcome the limitations imposed by the substrate composition, new approaches were suggested to ensure high and stable biohydrogen production from CSW.Novelty Statement: Studies that address the hydrogen evolution from real wastewaters are important to demonstrate the effective behavior of the fermentative process, considering the reactor resilience facing the variations in the OLR and compositional characteristics of the residue. To overcome these challenges, this study discusses solutions to achieve stable and continuous biohydrogen production from cassava starch wastewater, proposing alternatives to mitigate the problems related to its use as a substrate, such as a